Given, speed of a boat in still water (V) = 5 km/hr Let speed of the stream be V' km/hr. time taken to go 20km downstream (t) = distance travelled relative speed of the boat =
20
V+V′
=
20
5+V′
hours time taken to go 20 km upstream (t') = distance travelled ÷ relative speed of the boat =
20
V−V′
=
20
5−V′
hours Given that t′=3×t ⇒
20
5−V′
=3×
20
5+V′
⇒4V′=10 ⇒V′=2.5km/hr. So the speed of the stream is 2.5 km/hr.